It has been the custom for some time to consolidate the ballast of a railway track beneath a tie by inserting vibrating tamping blades into the ballast on either side of the tie, moving the tamping blades together so as to squeeze the ballast beneath the tie, and subsequently withdrawing the tamping blades.
More recently, it has been suggested that if respective plates were inserted in the crib on either side of the tie to be tamped and if those plates were vibrated by means of rotating out-of-balance weights, vibration would be transferred from the weights to the plates and through the ballast, whereby to cause the ballast to flow beneath the tie and become consolidated. It has been found that the "dynamic module of elasticity" of railway track ballast, which can be taken as a measure of the strength of the ballast structure, is greatly reduced when the ballast is excited with a vibration frequency of from 55 Hz. to 75 Hz.